Tuesday, June 22, 2010

Common Poorwills (Phalaenoptilus nuttallii) are widely distributed throughout western North America, but how many of you have seen one? They are strictly nocturnal and perfectly camouflaged in their daytime resting sites.

On a recent early June trip to eastern Washington, we drove up Robinson Canyon, west of Ellensburg, in the evening. Robinson Canyon is a riparian oasis in the midst of sagebrush shrub-steppe and ponderosa pine woodland. We arrived at about 8 pm at a gate in the fence that encloses part of the L. T. Murray Wildlife Recreation Area. As it got darker and darker, we enjoyed the sights and sounds of the local songbirds going to bed. A Western Wood-Pewee sallied after insects later than we expected a flycatcher to be out and about. Another one sang its dusk song nearby.

But we were after more elusive game. As it got dark, our quarry began to stir. At 9:12 pm, a poorwill called its name from the nearby shrubby and rocky hillside. Several others followed in succession, until a steady chorus of melodious ‘poorwill . . . . . . poorwill . . . . . . poorwill’ resounded from the hillsides. At 9:27 pm, we saw the silhouette of a bird flitting across the road. We started the car and pulled away from the roadside at 9:30, when it was entirely dark.

Very soon the headlights picked up a poorwill resting on the road. This bird eats large moths and beetles that it sees against the sky as it rests on open ground, and roads through its habitat often represent the most open terrain for a foraging bird, so we were taking advantage of this long-known way to get a look at poorwills. Their eyes shine brilliant orange in the light of the headlights.

Seven poorwills flushed from the road in the three miles of road we drove, and fortunately, as the goal of the evening was to get photos, two of them remained on the edge as we slowly drove by them, allowing shutters to click to complete satisfaction. The huge eyes, tiny beak (but the gape extends back behind the eyes), and tiny feet (almost useless for locomotion) were visible at close range.

Trying to find them in the daytime is another story. They sometimes perch right out in the open, on broken ground or rocks, but seeing one before you flush it is an art or science that is beyond me. I once chased one around, trying to get photos of it in broad daylight, and I could never see it before I flushed it, sometimes within a few meters. Their camouflage, I thus conclude, is perfect. Joe Marshall, an ornithologist who spent a lifetime studying nocturnal birds, called the Common Poorwill the most beautiful bird.

But poorwills are much more interesting than just being big-eyed, big-mouthed nocturnal insectivores. They were the first bird to be discovered “hibernating,” in deep torpor in a south-facing rock crevice in southern Arizona. Amazingly, this species is still the only bird known to spend long periods in torpor, remaining completely inactive for days during periods of low temperatures during winter. At those times, body temperature can drop to 5°C, oxygen consumption to <10% of normal. This has the earmarks of true hibernation, but it is not. Instead, it is a day-to-day phenomenon, not the deep winter sleep of marmots and other mammals. Poorwills may enter this state at any time of low temperatures, an energy-saving strategy that allows them to cope with periods when no insects are out and about.

The Black Swift (Cypseloides niger) is one of the more poorly known North American birds. Seen by most people in flight high overhead, its comings and goings are only poorly documented.

Swifts are aerial insectivores. Their very long wings, with extremely high aspect ratio (length/width), allow them to glide effortlessly or move forward at high speed by rapid wingbeats. They are large enough that their wingbeats are more obvious than those in smaller swifts such as Vaux’s. Not only can they fly for long periods while foraging, but they can fly long distances as well, This serves them not only for long-distance migration (they winter in northern South America) but also for daily foraging trips.

The diet of these birds is of course made up of flying insects. The prey is usually about a centimeter in length or less and may be flying ants, wasps, flies, beetles, leafhoppers or anything else that they can find in the air. Foraging is often very high, beyond the limits of human vision, but during cloudy and cool weather, the birds come much lower, often feeding over water bodies, where insects are usually present even in bad weather.

Because they are such superb fliers, the swifts can wander 80 km or more from their montane nesting areas in a single flight to look for food. They stay away for many hours, accumulating insect prey in a sticky mass in the throat. When they return to the nest, they feed the young by regurgitating this mass a bit at a time.

Nesting is always on cliffs, either on rugged coasts or in mountainous areas, usually behind a waterfall. The nests are built of moss (or moss and seaweeds for coastal nests). Because foraging is an uncertain business if you’re a swift, creating quite a challenge when feeding young, this species lays only one egg. Food delivery must be spotty, even with two parents providing it, as the young takes about seven weeks to leave the nest. Compare that with a nestful of five baby robins that fledge in two weeks!

While censusing birds at Port Susan Bay, Snohomish County, Washington ornithologist Steve Mlodinow observed numbers of these swifts at close enough range to get magnificent photos, perhaps the best ever taken in flight. These photos generated a lengthy discussion about why some birds had white tips to the feathers of their underparts and others didn’t. The consensus, aided by examination of specimens in the Slater Museum, is that the white-scalloped birds are females. In addition, the spotted birds had shorter tails. Sexual dimorphism is quite unusual in swifts, and its significance in Black Swifts is unknown.

Much is to be learned about this species, perhaps not easily. For example, no one has seen Black Swifts copulate; there is a challenge for an adventuresome field observer!

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About the Slater Museum

Our goals are to preserve and provide a collection of specimens to be used for research and education. Located in Tacoma, Washington, the Slater Museum is one of the region's significant repositories for bird, mammal, reptile, amphibian and plant specimens from the Pacific Northwest. In addition, we strive to educate the local community about nature and about the value of museums.